<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <meta http-equiv="X-UA-Compatible" content="ie=edge" />
  <title>Document</title>
  <style>
    html,
    body {
      height: 100%;
      margin: 0;
    }

    .a {
      position: fixed;
      top:0;
      left: 0;
      right: 0;
      bottom: 0px;
      border: 2px solid #f00;
      background-color: blue;
      margin-bottom: -1000px;
    }

    .b {
      position: absolute;
      bottom: 0;
      right: 0;
      width: 200px;
      height: 200px;
      /* margin-right: -100px;
      margin-bottom: -100px; */
      background-color: red;
      z-index: 1;    }
    .b.c{
      width: 100px;
      height: 100px;
      background-color: blue;
      z-index: 6;
    }
  </style>
</head>

<body>
  <div class="a">
    <div class="b">123</div>
    <div class="b c">123</div>
  </div>
</body>
<script src="./index.js"></script>

</html>